1. Field of the Invention
This invention pertains generally to transmission lines and networks, and more specifically to thick film type dissipating terminations that match the characteristic impedance of a transmission line.
2. Description of the Related Art
Transmission lines are used in a diverse array of electronic equipment to accommodate transmission of electrical or electronic signals. These signals may have a diverse set of characteristics, which might, for example, include direct or alternating currents, analog or digitally encoded content, and modulation of any of a diverse variety of types. Regardless of the characteristics of the signal, an ideal transmission line will conduct the signal from source to destination without altering or distorting the signal. Distance is inconsequential to this ideal transmission line, other than delays which might be characteristic of the transmission medium and the distance to be traversed.
At low frequencies and with direct current transmissions, many transmission lines perform as though they are nearly ideal, even over very great distances. Unfortunately, as the frequency of the signal increases, or as the frequency of component signals that act as a composite increases, the characteristics of most common transmission lines decay and signal transmission progressively worsens. This is particularly true when signals reach the radio frequency range or when transmission lines become longer. One common phenomenon associated with high frequency, long distance transmission lines is a loss of the signal's high frequency components and the introduction of extraneously induced interfering high frequency signals. Another common phenomenon is echo or line resonance, where a signal may be reflected from one end of the transmission line back to the other. This echo, in the case of analog voice signals, is commonly known as reverberation, which leads to the effect of one sounding like speech is emanating from within a barrel. The auditory reverberation within a barrel generates a sound similar to the sound after an electrical signal echoes within a transmission line. In the case of a digital pulse, the effect will lead to corrupted data, since additional pulses may be received that were not part of the original transmission, and reflected pulses may cancel subsequent pulses.
In a number of electrical and electronic fields, new circuitry is being developed that has ever increasing capability for higher frequencies. The benefits of these higher frequency components is realized in faster computer processing, in the case of data processing, or broader bandwidth transmissions which can carry more voice signals, more television and radio signals and other signals all over the same communications channel. However, as these communications channels utilize ever-increasing frequencies, the limitations of conventional transmission lines are accentuated. In the case of copper transmission lines, radiation from a signal conductor is dependent directly upon the transmission line length and relative proximity of adjacent signal conductors. So, for example, a long signal line adjacent to another long signal line causes trouble even at lower frequencies. The original telephone lines were twisted in a particular way to reduce signal coupling between separate telephone lines. This signal coupling was aptly referred to in the art by the phrase “cross-talk”, since signals from one telephone conversation would cross the lines into a different telephone line, resulting in talking which crossed the wires improperly. Cross-talk, as aforementioned, is dependent in part upon the spacing between adjacent signal lines. One method of reducing cross-talk is to increase spacing between lines. Unfortunately, another objective in the field of electronics is reduction of the size of components and systems. Simply increasing the spacing often results in greater expense, and also slower overall systems operation speeds—defeating the benefits which were otherwise attained by operating at higher frequencies. Another disadvantage of increased spacing comes from signal radiation. When a copper transmission line is made longer, the conductor will radiate and receive more high frequency energy. So, it is desirable to keep transmission lines shorter, not longer as might otherwise be dictated by cross-talk factors.
To prevent echo within a transmission line, it is possible to terminate the line with a device which is referred to in the art as an energy dissipating termination. The termination must have an impedance which is designed to match the characteristic impedance of the transmission line as closely as possible over as many frequencies of interest as possible. Transmission lines generally have an impedance which is based upon the inductance of the conductor wire, capacitance with other signal lines and ground planes or grounding shields, and resistance intrinsic in the wire. With an appropriate transmission line, the sum of the individual impedance components is constant and described as the “characteristic impedance.” To match the transmission line characteristic impedance over a wide frequency range, a termination must also address each of the individual impedance components. The effect of inductance is to increase impedance with increasing frequency, while capacitance decreases impedance with increasing frequency. Intrinsic resistance is independent of frequency.
In the particular field of data processing, transmission lines typically take the form of busses, which are large numbers of parallel transmission lines along which data may be transmitted. For example, an eight bit data bus will contain at least eight signal transmission lines that interconnect various components within the data processing unit. The data bus is actually a transmission line having to accommodate, with today's processor speeds, frequencies which are in the upper radio frequency band approaching microwave frequencies. These high frequency busses are, in particular, very susceptible to inappropriate termination and transmission line echo.
Terminations used for these more specific applications such as the data processor bus serve several purposes. A first purpose is to reduce echoes on the bus by resistively dissipating any signals transmitted along the bus. This first purpose is found in essentially all termination applications. A second purpose, more specific to data busses or other similar electronic circuitry, is to function as what is referred to in the art as a “pull-up” or “pull-down” resistor. The termination resistor will frequently be connected directly to either a positive power supply line or positive power supply plane, in which case the termination resistor is a “pull-up” resistor, or the resistor may be connected to either a negative or ground line or plane, in which case the resistor is referred to as a “pull-down” resistor. When no signal is present on the line, the voltage on the transmission line will be determined by the connection of the termination resistor to either a power supply line or a ground or common line. Circuit designers can then work from this predetermined bus voltage to design faster, more power-efficient components and circuits.
The prior art has attempted to address signal line termination in a number of ways which were suitable at lower operating speeds and frequencies, but which have not proven fully desirable as frequencies and components thereof increase. To address higher frequency signals, such as might be encountered in data processing computers, for example, smaller, more compact resistors are required. These resistors may be formed by one of several common processes. One such process is referred to as thin film, which might include vapor deposition techniques, sputtering, semiconductor wafer type processing, and other similar techniques. An example of a thin film component is found in U.S. Pat. No. 5,216,404 to Nagai et al. These thin film production techniques require special vacuum chambers that make sequential, continuous production difficult and expensive.
Thick film components, herein considered to be components that are formed from a layer of Cermet or dielectric material deposited upon a non-conductive substrate, are most commonly formed from screen printing techniques. Other processes may be used to form thick film components such as lamination, or from subtractive processes including etching. For the purposes of this application, thick films are defined as films formed when specially formulated pastes or inks are applied and fired or sintered onto a substrate in a definite pattern and sequence to produce a set of individual components, such as resistors and capacitors, or a complete functional circuit. The substrates can be either pre-fired or can be in a green un-fired state. The pastes are usually applied using a screen printing method and may typically have a thickness of from 0.5 to 1 mil or more, and are well known in the industry. Cermet materials are materials comprising ceramic or glass in combination with metal compositions, where the first three letters: CER & MET make the word CERMET.
TCR stands for Temperature Coefficient of Resistance, which is a measure of the amount of change in resistance over some temperature range. Sheet resistivity for the purposes of this disclosure is measured in the units of ohms per square. This will be considered herein to be the resistance of a 1 mil thick film of equal length and width.
Low TCR thick film resistors may be readily manufactured that are both durable and have excellent TCR. These resistors may have sheet resistances that vary from fractions of an Ohm to millions of Ohms per square with a TCR less than ±100 ppm/□C. The performance of these resistors is excellent, and they may be patterned and trimmed by laser ablation, mechanical methods or most simply by altering patterns in the screen to form very tight spirals. As a result of the many excellent characteristics of thick film materials, these materials are most desirably incorporated into transmission line terminations.
However, as frequencies increase, there is great demand to decrease the size of the components. For example, inductance increases with length. Therefore, to minimize inductance in the termination, signal lines should be kept as short as possible. Furthermore, shorter line lengths decrease the undesirable cross-talk described hereinabove. Stray capacitance should be minimized, since this stray capacitance is frequently variable with temperature due to temperature related variations in ordinary dielectrics.
In the prior art, transmission line terminations were initially constructed using large Cermet resistors which were formed by thick film techniques upon alumina (aluminum oxide) substrates. These components were then mounted into a circuit board in a Single-In-line Package (SIP) format. Several examples of these components may be found in U.S. Pat. No. 3,280,378 to Brady et al, U.S. Pat. No 3,346,774 to Brady, U.S. Pat. No 3,492,536 to Di Girolamo et al, each assigned to the present assignee, and also U.S. Pat. Nos. 4,654,628 and 4,658,234 to Takayanagi, all which are incorporated herein by reference. Due to the SIP format, one termination conductor must extend up from the substrate to a resistor, but additionally a second termination must extend up from the substrate and fully pass around the full length of the resistor, finally extending over the top of the resistor and terminating thereto. As a result, the effective conductor lead length found in these SIP components must be greater than the actual resistor length, and in most cases several times the resistor length. As lead length increases, so does line inductance, which then increases impedance to the high frequency components. When the inductance is too great from lead length, the termination device will not match the transmission line, and echoes will be generated as aforementioned, thereby corrupting data transmission or diminishing analog signal quality.
A second type of cermet termination has been developed, commonly referred to in the art as a “chip” type component. A flat substrate has resistors and terminations patterned thereon, and, unlike the SIP configuration, the chip component is laid flat onto the substrate. One example of the chip type component is illustrated in U.S. Pat. No. 5,379,190, the contents of which are also incorporated herein by reference. Since the chip is flat on the substrate, the chip component itself has shorter lead lengths. However, there are several very pronounced disadvantages of these chip components. A first disadvantage is the amount of circuit board real-estate which is consumed by the component. One of the reasons for the SIP configuration was to use as little circuit board surface area as possible. Circuit board real-estate is precious for two reasons. First of all, when a component uses more of the surface, wires must travel further to get past the component. As aforementioned, this means that the transmission line lengths are greater and radiation and the potential for cross-talk are greater. In addition, the circuit board itself has a price per unit area, which must be added to the component cost in an amount equal to the amount of surface area taken by the component. Another serious drawback is that, while the chip component lead lengths are shorter, the actual line lengths may, in fact, be no shorter, depending upon where the lines are routed from the chip into the circuit board pattern. In other words, while the distance on the chip is shorter, the actual total line length may not be any shorter.
To retain the size advantages of the SIP components, Seffernick et al in U.S. Pat. No. 5,621,619, assigned to the present assignee and incorporated herein by reference, developed a DIP configuration of reduced size and spacing smaller than ordinarily obtainable with solder paste and thick film Cermet compositions. Nevertheless, there is a continuing demand for even smaller and higher frequency transmission line terminations.
One method of component attachment which has proven beneficial in higher frequencies is the Ball Grid Array (BGA) package. In this package, connection between a printed circuit board and the BGA component is achieved through the use of a number of solder balls. These balls are not limited to placement around the periphery of the device, as was the case in the chip resistors of the prior art, but instead the BGA has terminations distributed in the array across the entire package. As a result, the printed circuit board real-estate may be consumed by the connection (the BGA), making this type of connection comparable in real-estate economy to the earlier SIP designs. In addition, the circuitry may be connected directly through the component substrate to the BGA, meaning that lead lengths may be limited solely to the thickness of the component substrate. These resultant leads are generally much shorter even than the leads found on chip components. Examples of these BGA type terminations are found in U.S. Pat. No. 4,332,341 to Minetti; U.S. Pat. No 4,945,399 to Brown et al; U.S. Pat. No 5,539,186 to Abrami et al; U.S. Pat. No 5,557,502 Banerjee et al; and U.S. Pat. No 5,661,450 to Davidson. Each of these patents illustrate various types of BGA components and packages, the contents and teachings which are incorporated herein by reference. While each of these patents illustrate various components, including termination resistor arrays and capacitors, none illustrate a high density thick film type termination network which has the benefits of resistors and capacitors integrated therein. Yet, such a device is needed in the art to provide the characteristics which are desired for many different types of transmission lines, including but not limited to the analog and digital lines described hereinabove.